Engineering Investigations
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Sanjib.Acharya
Yes, such a small die shrink should not matter. So I guess it was a pre-existing ...
pangliuliu
I'm wondering if a small die shrink: from 50nm to 40nm for the memory chip would ...
Controller PCB problem solved
Ravender Ajmani
1/24/2012 11:07 AM EST
The year was 1996 and it was my second year with this Hard Disk Drive company. I had joined them as an EMC Engineer to provide EMC support to the PCB design engineers. Within a year I was being called to help on almost every noise issue with the disk drives.
Our off-shore drive assembly plant reported a problem with the controller PCB, which was causing significant yield loss. A few suspect cards were sent to me to investigate. I was also informed that they had replaced all the modules (Processor, DRAM, DSP, Controller, Flash, etc.), and were suspecting the problem with the card fabrication process. I started sniffing the good and bad cards with my close-field EMI probe, and found out that the bad cards showed 40 MHz noise at the back side of the card. I traced the source of the noise to a couple of SRAMs, which were on the back of the card and had not been replaced by the assembly plant during troubleshooting. Inspection of the SRAMs showed that the SRAM lot number for the bad cards was different. The SRAM supplier company was contacted, and they informed us that they had been shipping die-shrink parts without obtaining our approval.
Once the problem was diagnosed, the use of die-shrink parts was stopped. Our commodity group got involved in the How and Why of the issue, which continued for a very long time.
Ravender Ajmani is the Principal Engineer at Hitachi Global Storage Technologies.
Our off-shore drive assembly plant reported a problem with the controller PCB, which was causing significant yield loss. A few suspect cards were sent to me to investigate. I was also informed that they had replaced all the modules (Processor, DRAM, DSP, Controller, Flash, etc.), and were suspecting the problem with the card fabrication process. I started sniffing the good and bad cards with my close-field EMI probe, and found out that the bad cards showed 40 MHz noise at the back side of the card. I traced the source of the noise to a couple of SRAMs, which were on the back of the card and had not been replaced by the assembly plant during troubleshooting. Inspection of the SRAMs showed that the SRAM lot number for the bad cards was different. The SRAM supplier company was contacted, and they informed us that they had been shipping die-shrink parts without obtaining our approval.
Once the problem was diagnosed, the use of die-shrink parts was stopped. Our commodity group got involved in the How and Why of the issue, which continued for a very long time.
Ravender Ajmani is the Principal Engineer at Hitachi Global Storage Technologies.
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Simplifried
1/25/2012 3:55 AM EST
Ravender, what prompted you to look for EMI?
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WKetel
1/27/2012 8:03 PM EST
When everything else checks out right, then you look for noise, and often that noise is RFI. I wonder if his employers purchasing people approved the substitution of the "die-shrunk" parts as part of a cost reduction, or if the supplier just made the change to improve their profits. IN many cases it happens that "equivalent parts" are not close enough to function as required, and so problems arise when they are used.
It was good detective work indeed.
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AussieNeil
1/27/2012 10:14 PM EST
I've seen the same problem with a LM324 op amp die shrink. Reduced yield problems were directly correlated with a die shrink (and no, we weren't informed of the change). In this case, the die shrink was physically obvious (even if the electrical performance change wasn't immediately so) as the IC was bought in slice form and cut, glued and wire bonded into the module.
In this case, poor layout practice was a factor (a non-inverting input track ran parallel to an output track for perhaps a couple of centimetres), but modules using the larger die didn't break into oscillation.
Obviously the smaller transistors in the shrunk die had a higher cut-off frequency and the internal compensation probably didn't work with as much margin as in the larger, "electrically equivalent" IC.
I can understand the op amp manufacturer's point of view in not informing us of the change given the part was electrically equivalent and the die shrink part would probably have been approved on the basis of a desktop review anyway...
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sharps_eng
1/29/2012 4:28 AM EST
@AussieNeil: 'a non-inverting input track ran parallel to an output track for perhaps a couple of _centimetres_' ; doesn't seem like a die SHRINK to me...!?
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Sanjib.Acharya
1/29/2012 9:57 AM EST
If the manufacturer has not informed the change to a die-shrink part, I guess the chip specifications (mostly the timing specs) published in datasheet has not changed. In that case the only change was in the impedance (especially the inductance) of the package pins. Would this make such a huge impact on the board performance unless there was inherent design issue on board, which was on the border line?
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pangliuliu
2/20/2012 1:59 AM EST
I'm wondering if a small die shrink: from 50nm to 40nm for the memory chip would lead to EMC issues (Radiated Emission?) The specification sheets are exactly same from the manufacturer.
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Sanjib.Acharya
2/26/2012 3:04 AM EST
Yes, such a small die shrink should not matter. So I guess it was a pre-existing quality issue, which was missed from previous qualification testing?
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